BACKGROUND Ovarian stimulation by gonadotrophin treatment exerts negative effects on implantation and embryonic development. We investigated whether gonadotrophin treatment affects VEGF(120) mRNA expression during the peri-implantation period. METHODS Two groups of adult female CD1 mice were used: the hormone-treated group was injected i.p. with urinary human FSH (5 IU in 0.1 ml saline) and urinary HCG (5 IU in 0.1 ml saline). Spontaneously ovulating mice served as controls and received saline injections. The pregnant mice were killed on embryonic development (ED) days 0, 3, 4, 5 and 6 (day of vaginal plug detection is considered as ED0). The uteri with the implanted embryos were processed for in-situ hybridization for VEGF(120). A separate group of control and hormone-treated pregnant mice were allowed to give birth. Litter size, birthweight and length of gestational period were noted. RESULTS Gonadotrophin treatment decreased VEGF(120) mRNA levels, delayed implantation, reduced the size of the embryo implantation site on ED5 and ED6 and prolonged the gestational period. CONCLUSIONS Gonadotrophin treatment reduces VEGF(120) expression which may have serious consequences for normal embryonic development. The present data cannot establish whether this effect is a cause or consequence of delayed implantation.